Aqueous Lyotropic Mesophase Behavior of Gemini Dicarboxylate Surfactants Swollen with -Decane.

We report detailed small-angle X-ray scattering (SAXS) studies of the impact of variable -decane loadings on the lyotropic liquid crystalline (LLC) phase behaviors of homologous bis(tetramethylammonium) gemini didecanoate surfactants , which derive from dimerizing decanoic acid through its α-carbon with hydrocarbyl linkers -(CH)- where = 3, 4, 5, and 6. amphiphiles with = 3 or 5 exhibit a strong propensity to form normal double gyroid (G) LLC network mesophases over wide surfactant hydration ranges, as compared to homologues with = 4 or 6. On swelling aqueous LLC mesophases with up to 35 wt % -decane, we demonstrate that odd-carbon linked surfactants ( = 3 or 5) form G and normal double diamond (D) phases over wide water concentration windows with = 22-100 °C. Complementary studies of decane-swollen ( = 4 or 6) aqueous LLCs instead demonstrate significantly diminished network phase stability, in favor of hexagonally-packed cylinder phases and a zoo of complex quasispherical micelle packings, which include micellar C14 and C15 Laves phases ( and symmetries, respectively) and high-symmetry hexagonally close packed (HCP) and body-centered cubic (BCC) arrangements. These rich phase behaviors are rationalized in terms of linker length parity-dependent surfactant conformations and the delicate free energy balance that guides the packing of these geometrically anisotropic amphiphiles by minimizing unfavorable water-hydrophobic contacts, maximizing ionic surfactant-headgroup counterion solvation with minimal local variations, and maximizing electrostatic cohesion within these supramolecular assemblies.